This application claims the priority of Japanese Patent Application No. 2001-99010 filed on Mar. 30, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an imaging optical system for an oblique incidence interferometer; and, more specifically, to an imaging optical system, disposed between a surface to be inspected and an imaging surface constituted by an image pickup device, for forming an image of the surface to be inspected having interference fringes superposed thereon onto the imaging surface.
2. Description of the Prior Art
Oblique incidence interferometers have conventionally been known, in which coherent light is made obliquely incident on a surface to be inspected, so as to lower the sensitivity in measurement, thereby making it possible to measure the surface form having a relatively large degree of irregularities. Known as a typical configuration of the oblique incidence interferometers is one shown in FIG. 4, for example.
This oblique incidence interferometer is configured such that coherent light is made incident on a wavefront splitting means 102. The wavefront of incident light is split into two directions. One of the resulting luminous fluxes is made obliquely incident on a surface to be inspected 2a, so as to become measurement light reflected thereby, whereas the other luminous flux is used as reference light. The measurement light and reference light are made incident on a wavefront combining means 104 so that their wavefronts are combined together. Interference fringes generated by the optical interference between the measurement light and reference light emitted in the same direction from the wavefront combining means 104 are captured by a video camera 108 by way of an imaging lens 106, and the form of the surface to be inspected 2a is measured according to thus obtained interference fringe image.
In such an oblique incidence interferometer, however, the optical path length to the imaging lens 106 varies depending on positions on the surface to be inspected 2a, so that trapezoidal distortions may occur in the interference fringe image captured by the video camera 108, whereby the form of the surface to be inspected 2a may not be measured accurately. Though it is possible to eliminate trapezoidal distortions by electronic image processing, arithmetic operations for compensating for the resolution of image pickup device are complicated, and requirements for tolerances for apparatus become severer, whereby this approach is unfavorable.
Known as an oblique incidence interferometer solving such a problem is one shown in FIG. 5. This apparatus is configured such that, an interference fringe viewing screen 110 is disposed at a position conjugate with a surface to be inspected 2a, whereas interference fringes superposed on the image of the surface to be inspected formed on the interference fringe viewing screen 110 are captured by a video camera 108 arranged perpendicular to the screen 110. In this drawing, an imaging lens 106 is disposed such that its first focal point is positioned at the surface to be inspected 2a, a collimator lens 112 is arranged afocal with respect to the imaging lens 106, and the interference fringe viewing screen 110 is disposed at a second focal position of the collimator lens 112. Such a configuration makes it possible to form an image of the surface to be inspected having the same size as the surface to be inspected with substantially the same longitudinal and lateral magnifications with respect to the surface to be inspected, whereby trapezoidal distortions of the image are prevented from occurring.
Employing such a configuration, however, may be problematic in that the total length of the interferometer increases. Since the interference fringe viewing screen 110 is disposed at the position conjugate with the surface to be inspected 2a with respect to the imaging lens 106 and collimator lens 112, the screen 110 must have the same size as that of the surface 2a, which may hinder the apparatus from being made compact, though trapezoidal distortions of the image of the surface to be inspected are prevented from occurring.
Therefore, imaging optical systems of oblique incidence interferometers have been desired to be able to capture images of interference fringes without trapezoidal distortions, and reduce the screen size, so that the apparatus can be made smaller. Here, the image of surface to be inspected having interference fringes superposed thereon to be captured is desired to be an in-focus image having substantially the same longitudinal and lateral magnifications with respect to the surface to be inspected, which is not required to be corrected by arithmetic operations.
In order to overcome such a problem, it is an object of the present invention to provide an imaging optical system for an oblique incidence interferometer, which can form an image of a surface to be inspected having substantially the same longitudinal and lateral magnifications with respect to the surface to be inspected onto an imaging surface while being able to reduce the size of the apparatus.
The present invention provides an imaging optical system for an oblique incidence interferometer in which a part of light source light is turned into parallel light so as to become measurement light obliquely incident on a surface to be inspected in a sample, a part of the remainder of the light source light is used as reference light constituted by parallel light, and the measurement light and the reference light are combined together and caused to interfere with each other so as to superpose interference fringes onto an image of the surface to be inspected formed on an imaging surface;
the imaging optical system comprising:
a first imaging optical system comprising two groups of telecentric lenses G1, G2 having respective focal lengths different from each other, the telecentric lens G1 having a longer focal length and the telecentric lens G2 having a shorter focal length being arranged afocal to each other successively from the surface to be inspected side;
an intermediate imaging surface; and
a second imaging optical system comprising two groups of telecentric lenses G3, G4 having respective focal lengths different from each other, the telecentric lens G3 having a longer focal length and the telecentric lens G4 having a shorter focal length being arranged afocal to each other successively from the surface to be inspected side;
wherein a first image of the surface to be inspected, reduced from the surface to be inspected, having a deformed aspect ratio with respect to the surface to be inspected is formed on the intermediate imaging surface by the combined light of the measurement light and reference light by way of the first imaging optical system; and
wherein the second imaging optical system is arranged with respect to the intermediate imaging surface such that the first image of the surface to be inspected is focused by the second imaging optical system onto the imaging surface as a second image of the surface to be inspected which is reduced from the first image of the surface to be inspected and corrected so as to have substantially the same aspect ratio as that of the surface to be inspected.
The second imaging optical system may form on the imaging surface an image of the surface to be inspected caused by scattered light transmitted through the intermediate imaging surface.
The second imaging optical system may form on the imaging surface an image of the surface to be inspected caused by scattered light reflected by the intermediate imaging surface.
Preferably, the imaging optical system satisfies the following conditional expressions (1) to (3):
xe2x80x83tan xcex82=xcex21 tan xcex81xe2x80x83xe2x80x83(1)
tan xcex84=xcex22 tan xcex83xe2x80x83xe2x80x83(2)
cos xcex81/cos xcex82=cos xcex83/cos xcex84xe2x80x83xe2x80x83(3)
where
xcex81 is the angle formed between a normal of the surface to be inspected and the optical axis of the first imaging optical system;
xcex82 is the angle formed between a normal of the intermediate imaging surface and the optical axis of the first imaging optical system;
xcex83 is the angle formed between the normal of the intermediate imaging surface and the optical axis of the second imaging optical system;
xcex84 is the angle formed between a normal of the imaging surface and the optical axis of the second imaging optical system;
xcex21 is the absolute value of magnification of the first imaging optical system; and
xcex22 is the absolute value of magnification of the second imaging optical system.
Preferably, the intermediate imaging surface is constituted by a diffuser rotating with a center of rotation located on the intermediate imaging surface or an extended plane thereof.
The diffuser may be a ground glass sheet, a holographic screen, or a liquid crystal screen using a dynamical scattering mode.